This invention relates to a method of producing nitrogen by liquefying and rectifying air by using, for example, a single rectification column, and more particularly to a method of improving the yield of nitrogen by providing a gas circulating system.
A method using a single rectification column is employed most widely as one for producing nitrogen by means of low temperature liquefaction and rectification of air and the process has the construction whose system outline is shown in FIG. 1.
In this process, raw air, compressed to 5 to 10 Kg/cm.sup.2 abs by a raw air compressor 1, is fed into an adsorber 4 for removal of carbon dioxide and water contained therein. The resultant cleaned air is then fed into a heat exchanger 6 wherein it is cooled to a temperature close to its liquefying point. Thereafter the cooled air is fed into the bottom portion of a single rectification column 8 having a condenser 11 disposed in a column overhead wherein the rectification of the introduced air is effected so as to separate high purity nitrogen gas from the same at the column overhead as well as a liquid air containing 30 to 40% of oxygen by volume at the column bottom. This oxygen-enriched liquid air passes through a pipe 9, and is expanded and cooled by being reduced to a pressure of 2 to 6 kg/cm.sup.2 abs. by an expansion valve 10. The resultant cooled air, fed into a cooled flow channel 11a of the above-mentioned condenser 11, is heat exchanged with the above-mentioned high purity nitrogen gas which flows from the overhead of the rectifying column 8 and then branches via a pipe 12 into a flow channel 11b of the condenser 11, so that the heat-exchanged nitrogen gas is liquefied and again flows back to the overhead of the single rectifying column 8 as a reflux while the oxygen-enriched liquid air itself is vaporized into a pipe 16.
The oxygen-enriched air at a temperature of -190.degree. to -160.degree. C. which is vaporized in the condenser 11 then enters a flow channel 6c of the above-mentioned heat exchanger 6 wherein it is warmed to -160.degree. to -90.degree. C. by heat exchange with the above-mentioned raw air in a flow channel 6a. The resultant warmed oxygen-enriched air enters an expansion turbine 17 wherein its pressure is lowered by expansion to produce the required cold. The oxygen-enriched air cooled at -190.degree. to -160.degree. C. enters a flow channel 6f of the above-mentioned heat exchanger 6 wherein it provides cold to the raw air flowing in the flow channel 6a, whereupon it is discharged out of the system. The remainder high purity nitrogen gas which flows from the overhead of the rectifying column 8 via a pipe 12 and which is branched into a pipe 15 flows through a flow channel 6b of the above-mentioned heat exchanger 6, where it is restored to normal temperature and is withdrawn out of the system as a product gas. A nitrogen production method in accordance with this process has advantages in that the entire system requires a less complicated construction and that a product nitrogen gas is not required to be recompressed although the gas withdrawn from the overhead of the rectifying column has merely to be restored to normal temperature, thereby enabling its direct delivery to consumers. However, this method has the disadvantage that electric power consumption becomes large due to the low yield and the large quantity of raw air which needs to be compressed.
From this viewpoint, as a method of reducing these disadvantages and improving the yield of product nitrogen, a method has been proposed wherein the quantity of vaporized gas is increased by a reboiler at the bottom portion of the rectifying column by providing a nitrogen cycle, and simultaneously this circulating nitrogen is fed into the rectifying column overhead to increase the reflux quantity, thereby improving rectification efficiency. In accordance with this method, in the system indicated by the broken line in FIG. 1, product nitrogen gas branches into a pipe 22 and is then compressed to about 15 Kg/cm.sup.2 abs. by a circulating compressor 23 and is removed of compression heat. Thereafter the resultant cooled nitrogen gas is fed into a flow channel 6d of the foregoing heat exchanger 6 wherein it is cooled to a temperature close to its liquefying point and then, via a pipe 25, into a reboiler or evaporator 26 provided at the bottom portion of the above-mentioned rectifying column 8 for the evaporation of the oxygen-enriched liquid air existing at the bottom portion of the rectifying column 8. The nitrogen gas itself is liquefied and flows through a pipe 27 through an expansion valve 28 whereby it is expanded and cooled, and is then fed into the overhead of the rectifying column 8 from which it flows down in the column as a reflux liquid.
However, the above described method normally requires a high pressure of 15 Kg/cm.sup.2 abs. or more as a nitrogen cycle pressure for operating the reboiler 26 and the quantity of gas required for each cycle in order to obtain a certain amount of evaporation or reflux becomes larger due to the use of a region wherein latent heat of evaporation is small. Furthermore, it is difficult to employ a centrifugal type as a compressor which is excellent in continuous running performance for circulating nitrogen, due to the high pressure as described above, and there are also may cases where operators have been forced to use reciprocating or centrifugal type compressors in a region of low efficiency, so that sufficient savings on power consumption are not attained.
This invention aims to improve the low product yield which has been a disadvantage of the conventional single rectifying column method described above, and to allow electric power consumption rate to be reduced.
In view of these and other objects, the present invention provides a nitrogen production method wherein air is compressed, is removed of water and carbon dioxide contained therein, and is simultaneously cooled to a temperature close to the liquefying point, and the resultant cleaned and cooled air is fed into a rectifying column for rectification so that high purity nitrogen is withdrawn from the rectifying column overhead; and wherein the oxygen-enriched liquid air withdrawn from the rectifying column bottom is expanded and fed into a condensation step wherein it becomes a source of reflux producing cold in the above-mentioned rectifying column, and then a cold is produced by adiabatically expanding the vaporized gas, after which heat exchange with raw air is performed.
In this method, there is provided a closed circulating system wherein a circulating gas which is compressed is cooled by heat exchange with a return gas of the circulating gas, and the resultant cooled gas is fed into a reboiler at the bottom of the above-mentioned rectifying column wherein it vaporizes a liquid in the bottom of the rectifying column, and after the compressed circulating gas itself is liquefied through the above described step and expanded, it is fed into the above-mentioned condenser wherein it is vaporized by heat exchange with the high purify nitrogen from the above-mentioned rectifying column, and further the resultant vaporized gas is restored to normal temperature by heat exchange with the above-mentioned compressed circulating gas and is then subjected to recompression for continued circulation.
Since the amount of reflux in the rectifying column can be increased by the provision of the above-described circulating system, the rectification conditions are improved and the amount of raw air becomes smaller as compared with the amount of product nitrogen, that is, the yield of product nitrogen is increase. In this case, the amount of raw air used in the conventional method shown in FIG. 1 which has no circulating system may be substantially equal to the total amount of raw air and circulating gas used in the case of incorporating a circulating system, or alternatively the total amount of raw air and the circulating gas may be smaller. The compression ratio of the raw air compressor is mainly determined in accordance with the pressure required for the product nitrogen, and in the case of the prior art the ratio is normally a value of 6 or more. The compression ratio of a compressor for a circulating system is fundamentally determined in accordance with the temperature difference between the top section and the bottom section of the rectifying column thereof, and the ratio normally becomes a value of 3 or less in view of the temperature difference required for the heat exchange between the reboiler and the condenser as well as the pressure loss in the circulating system. Thus, a fairly large reduction in the electric power needed for the raw air compressor is obtained by providing this circulating system as compared with an increase in electric power for the circulating compressor, thus resulting in a considerable reduction in the total power needed.
However, in the case of FIG. 1 in which the nitrogen cycle is employed the pressure of circulating system must be high and a large amount of circulating gas is needed because the latent heat of nitrogen is small. Thus, in the present invention, the circulating system has a closed cycle and the present invention uses as a circulating gas a gas which has a higher boiling point and larger latent heat at pressurized state than nitrogen.
In the present invention, the circulating gas may be composed of a single or a mixed gas, each having its boiling point at an intermediate point between the respective boiling points of nitrogen and oxygen.
Preferably, the circulating gas is, in the present invention, argon as a single component gas and a gas containing at least two components of nitrogen, argon and oxygen, for example, air as a mixed gas. Such a gas has a higher boiling point and a greater latent heat than nitrogen, thereby enabling the pressure in the circulating system and the circulating amount to be kept at low levels, so that the power consumption of the compressor can be further reduced.